A soil sample with a pH of 4.5 is said to be acidic. pH is a measure of the acidity or alkalinity of a substance, and it ranges from 0 to 14. A pH of 7 is considered neutral, below 7 is acidic, and above 7 is alkaline. Therefore, a soil sample with a pH of 4.5 is acidic, which means it has a higher concentration of hydrogen ions and a lower concentration of hydroxide ions. This acidic condition affects the nutrient availability and uptake by plants and can affect crop growth and yield.